Heat exchange

ABSTRACT

A thermoconductive, fluid-confining tube has a plurality of thermoconductive elements each having opposed major surfaces, in thermoconductive contact with the outer surface of the tube, each element having a major dimension extending outwardly from the outer tube surface and a major dimension extending substantially parallel to the axis of the tube, and a smallest dimension, and is adapted for heat exchange with a second fluid directed along the surface of the tube, in an overall flow direction substantially parallel to the smallest dimension of the elements; the thermoconductive elements are spaced apart in the flow direction and define open areas perpendicular to the flow direction with the total per cent of open area perpendicular to the flow direction being less than the total per cent of open area in the plane perpendicular to the tube axis.

United States Patent [451 Sept. 19, 1972 Gerstmann HEAT EXCHANGE [72]Inventor: Joseph Gerstmann, Sudbury, Mass. [73] Assignee: Steam EngineSystems Corporation,

Newton, Mass.

[22] Filed: Feb. 3, 1970 [21] Appl. No.: 8,280

[52] US. Cl ..165/163, 122/250, 122/367 C, 165/172, 165/183 [51] Int.CL; ..F28f 1/14 [58] Field of Search ..l65/l63,18l, 183,164,125,165/172; 122/250, 7, 367, 367 C, 367 R, 367 A, 367 C, 225

[56] References Cited UNITED STATES PATENTS 620,994 3/1899 Teste..122/367 C X 1,993,850 3/1935 Mclntire ..122/367 RX 2,286,271 6/1942Higham ..'.....l65/l83 X 3,289,756 12/1966 Jaeger ..l65/180 X 3,397,4408/1968 Dalin ..l65/184 X FOREIGN PATENTS OR APPLICATIONS 1,028,0705/1966 Great Britain ..122/367 llllllll |1l|ll||| 1 Hill! 1| llllllllinmmu 866,348 2/1953 Germany 165/1 83 Primary Examiner-Albert W. Davis,Jr. Attorney-Edgar H. Kent 7 1 ABSTRACT A thermoconductive,fluid-confining tube has a plurality of thermoconductive elements eachhaving opposed major surfaces, in thermoconductive contact with theouter surface of the tube, each element having a major dimensionextending; outwardly from the outer tube surface and a major dimensionextending substantially parallel to the axis of the tube, and a smallestdimension, and is adapted for heat exchange with a second fluid directedalong the surface of the tube, in an overall flow directionsubstantially parallel to the smallest dimension of the elements; thethermoconductive elements are spaced apart in the flow direction anddefine open areas perpendicular to the flow direction with the total percent of open area perpendicular to the flow direction being less thanthe total per cent of open area in the plane perpendicular to the tubeaxis.

7 Claims, 7 Drawing Figures llllllllll PATENTEDSEP 19 m2 SHEET 2 BF 2 34380 FIG 5 FIG 3 FIG 4 llllll lll|- HEAT EXCHANGE This invention relatesto heat exchangers, and particularly to heat exchanger configurationsuseful for gas-fluid heat exchange, such as in vapor (e.g. steam)generators producing superheated vapor.

Someforms of heat exchanger configurations for steam generatorsconventionally employ circular tubing, having helically wound finswrapped around the periphery of the tubing to form an extended heatexchange surface, with the tubing coiled into a geometrical shape suchas a cone or cylinder. A first fluid flowing through the tube is heatedby second hot fluid (typically combustion gases) flowing generallyradially of the geometrical shape. Several concentric coils are commonlyemployed, although occasionally a single coil or bank is employed.

While there are many applications for which conventionally finned tubingis ideally suited, there are applications for which it is unsuitable orinefficient. It frequently occurs that a heat exchanger is to bedesigned which has a relatively large frontal area. Such is the case forinstance, in the design of direct fired heat large, thereby causing alow gas velocity through that area. Since the coefficient of heattransfer is dependent on gas velocity, a relatively low heat transfercoefficient results, which must be compensated for by a larger heattransfer surface. The requirement for large heat transfer surfacenecessitates utilizing more banks of finned tubing, which causes theheat exchanger to be quite large and heavy.

' Similarly, the low gas velocity results in a low pressure drop. Thispressure drop is frequently much below that which ordinarily would betolerated. The need exists, therefore, for a compact heat exchangerextended surface which promotes high coefficients of heat transfer atlow gas velocities, even though the increased heat transfer may beprovided at the expense of higher pressure drop.

Another disadvantage of conventional finned tubing concerns thedifficulty with which it is coiled. Because of the danger of crushingthe fins on the winding mandrel, it must be specially made to supportthe fins during winding. The radius of the coil is also limited by thetips of the inner fins touching if the winding radius is small or if thefin height is large.

A further problem is encountered whenonly a single bank of tubes isemployed. In this case, the minimum gas flow resistance occurs at thefin tips between adjacent tubes. Therefore a larger portion of the gasflows through this region, essentially by-passing the bulk of theextended surface. This has the effect of reducing the heat transferperformance proportionally below what it would be if there were noby-passing.

It is therefore an object of this invention to provide heat exchangersof increased efficiency, which are compact, and of conventionalgeometrical shapes.

Another object is to provide novel improved fluid carrying heat exchangetubing which not only has efficient extended surfaces, but is alsoeasier to manipulate, and forms more compact heat exchangers.

A further object is to provide vapor generators of improved efficiencyand compactness, which are suitable for a wide range of industrialapplications, ranging from water heaters to vapor generators forexternal combustion engines used in land, water or air conveyances.

The invention features, in one aspect, a heat exchanger comprising athermoconductive tube for confining a first fluid, a heat exchangematrix comprising a plurality of thermoconductive elements each havingopposed major surfaces, in thermoconductive contact with the outersurface of the tube, each element having a major dimension extendingoutwardly from the outer tube surface and a major dimension extendingsubstantially parallel to the axis of the tube, and a smallestdimension, and structure directing a second fluid along the surface ofthe tube in an overall flow direction substantially parallel to thesmallest dimension of the elements, the thermoconductive elements spacedapart in the flow direction and the matrix having open areasperpendicular to the flow direction, with the total per cent of openarea perpendicular to the flow direction being less than the total percent of open area in the plane perpendicular to the tube axis. In apreferred embodiment, two sets of oppositely extending thermoconductiveelements have elements arranged in parallel to one another with elementsextend-' ing in the same outward direction from the tube terminating inthe same plane.

In another aspect, the invention features a thermoconductive tube forconfining a fluid for heat exchange with a second fluid external to thetube. The tube has a plurality of thermoconductive elements inthermoconductive contact with its outer surface, having opposed majorsurfaces, the elements being arranged with all the major surfaces inparallel and in two sets, each' set extending in an opposite outwarddirection from the tube, and the elements being spaced apart in theaxial direction to define therebetween open areas, the total per cent ofopen area in the plane of the major surfaces being less than the percent of open area in the plane perpendicular to the tube axis. In apreferred embodiment, the elements are spaced apart and staggered in theflow direction so that upstream elements overlap downstream elements.Preferably, both ends of each upstream element overlaps a downstreamelement; and the major dimensions of the elements extending in the samedirection outwardly of the tube terminate in the same plane. In apreferred embodiment, the elements in each. set are arranged in axiallyextending rows, each row being coplanar with a row in the opposite set;the elements are secured to the outer tube surface through axiallyextending thermoconductive base strips secured to the outer tubesurface, each base strip supporting at least one row; base stripssupporting only one row do not extend all the way to the next row; and,a base strip supporting two adjacent rows is formed integrally with themember from a substantially U-shaped thermoconductive structure, theside walls of which are periodically slotted to define a row of elements(the slots between adjacent elements being preferably less than thewidth of the elements between slots).

In accordance with the present invention, the second fluid is thereforemade to flow the hard way through the thermoconductive elements,inducing intense turbulence and eddies which greatly increase j factorsover those for more streamlined flow (the j factor being a measure ofthe intrinsic heat transfer capability of a surface). As a result, heatexchangers of increased efficiency are achieved which are still compactand hence economical.

In a further aspect, the invention features a heat exchanger in which atube having two sets of thermoconductive elements as described isarranged in a coiled configuration with elements on adjacent windingsextending toward one another and touching or, at most, having a verynarrow passage between them. The fluid is thereby forced to follow asinuous route along the extended surfaces and between adjacent elements,leading to a high heat exchange efficiency with a not intolerably largepressure drop across the coiled tube. In one preferred configuration,the tube is (preferably, helically) coiled to form one or a series ofconcentric nested members of circular cross section (e.g., cones orcylinders), with the flow direction of the second fluid substantiallyradial to the member, and the tube defining a continuous first fluidflow path within which, for example, an entering feed liquid may beheated to exit as superheated vapor (e.g., water to superheated steam).

Other objects, features and advantages will be apparent to one skilledin the art from the following description of a preferred embodiment ofthe present invention, taken together with the attached drawingsthereof, in which:

FIG. 1 is an elevational view partially broken away, of a heat exchangeapparatus embodying the present invention;

FIG. 2 is an end view, partially in section, of the apparatus of FIG. 1;

FIG. 3 is a plan view of a segment of heat exchange tube embodying thepresent invention;

FIG. 4 is a plan view of the heat exchange tube of FIG. 3, taken at 90to the view of FIG. 3;

FIG. 5 is a sectional view of the tube of FIG. 3, along line 5-5 of FIG.4;

FIG. 6 is a sectional view of another tube configuration embodying thepresent invention; and,

FIG. 7 is a magnified view of a portion 7-7 of the apparatus of FIG. 1.

The figures show a heat exchange apparatus 10, including a housing 12,an inlet manifold 14 (which may enclose a suitable burner (not shown)),a fluid inlet 16 to manifold 14, an outlet manifold 17, and a fluidoutlet 18. Concentric heat exchange members 20, 22, 24 (illustratively,cylindrically shaped) are formed of a continuous helically woundthermoconductive tube 26, which has an interior fluid confining chamber28 of cylindrical cross section. Member 20 includes a fluid inlet 30which is in communication through tube 26 with a fluid outlet 32 inmember 24. Where apparatus is a steam generator, water enters as aliquid at inlet 30 and exits as superheated vapor at outlet 32.

Referring particularly to FIGS. 3-6, tube 26 has a plurality ofsheetlike thermoconductive fins 34, in thermal contact with its outersurface 36. Each fin has opposed major surfaces 38a, 38b,(illustratively shown as planar and providing the two major dimensionsof the fin) parallel to the axis of tube 26, and a smaller dimensionperpendicular to surfaces 38a, 38b. The fins are arranged, in coplanarparallel rows, in two sets 42a, 42b which extend in opposite directionsfrom the tube. The axial distance, d, (FIG. 3), between adjacent fins isless than the width, w, of major surfaces 38, and the heights, h, (FIG.4), of fins in each row are adjusted so that all fins of each set havecoplanar end walls 44. For any single row, therefore, the per cent ofopen area between the end walls 44 and tube surface 36, between adjacentfins 34, for fluid flow parallel to the smallest dimension of the fins,is less than 50 per cent of the total area including the fins and thespacing between adjacent fins. This is considerably less than the percent of open area in the direction parallel to the tube axis (i.e., in aplane perpendicular to the tube axis). The spacing between adjacent rowsis dependent, inter alia, on the tolerable pressure drop across thefins, as well as the heat exchange efficiency of the individual fins (ahigh efficiency limiting the required number), and weightconsiderations. AS seen in FIG. 5, the outer two rows of fins areadvantageously located as close to the tangents t of the tube parallelthereto as is consistent with equal spacing between rows.

The thermoconductive fins 34 may be secured to the outer surface 36 oftube 26 by conventional methods, such as welding, brazing and the like.A particularly rapid fabrication process is afforded if the fins areformed integrally with a base strip, which may support one (base strips48 of FIG. 4) or two (base strips 50 of FIG. 6) or even more rows offins. The integrally axially extending thermoconductive structures 52,54 defined thereby may be formed of an L-shaped plate, as in structure52, having a height h, slotted at spaced equal intervals along itslength to define a plurality of spaced thermoconductive fins of height hconnected by a base wall of height 0, 0 being most preferably equal tothe plate thickness. For the structure 54 of FIG. 6, a U- shaped plateis similarly slotted along its length, except that the slots in eachupstanding side 57 are staggered to produce adjacent rows of overlappingthermoconductive fins.

A tube formed as described is then wound about an axis approximatelyperpendicular to the tubeaxis and parallel to surfaces 38a, 38b, so thatthe end walls 44 of the opposed thermoconductive fins of adjacent coilsare touching or nearly touching. These end walls may be secured to oneanother, again by welding, brazing, or the like, at selected points orthroughout the apparatus, to lend integrity to the apparatus. As seen inFIG. 7, which is a magnified view of the encircled portion of the heatexchanger of FIG. 1, the parallel arrangement of the fins on the tubeallows concentric windings to be located very close together, eventouching, so as to increase the compactness of the heat exchanger andprevent by-passing of the gases over the tips of the fins. The tube orfins of one winding may be selectively spot welded (e.g., at selectedpositions 60, 62) to the thermoconductive fins of an adjacent winding ifdesired. As shown in FIG. 7, a winding 26a of member 22 has its fins 34aspot welded to the fins 34b of adjacent winding 26b, and to the fins34c, 34d of the windings 26c, 26d of the member 20.

In operation, which will be described for illustrative purposes for asteam generator, fuel and air are fed through inlet 16 and are combustedin a suitable interior burner; the resultant hot fluids traverse members20,

22, 24 in an overall flow direction parallel to the smallest dimensionof the fins, and thus substantially perpendicular to the surfaces 38a,38b but in a localized sinuous flow pattern, as shown diagrammaticallyby the dotted lines in FIG. 3. The actual flow pattern is morecomplicated than that shown, but the diagram illustrates the prolongedcontact between hot gas and heat exchange surfaces throughout thetraverse of the gas through the cylindrical heat exchange members. Thecooled gas passing to manifold 17 exits through gas outlet l8.

Feed liquid enters at fluid inlet 30, of the outer member 20, graduallybecoming hotter as it contacts hotter gas zones radially inwardly of theheat exchanger, and exits as superheated vapor through outlet 32.

Other embodiments, within the following claims, will occur to thoseskilled in the art. For example, two sets of thermoconductive fins, eachonly a few fins wide, could be arranged each on a semi-cylindrical basewall corresponding to the outer diameter of the tube, and the tips ofthe fins joined to form an integral structure in which fins extendbetween opposed semi-cylindrical end walls. A plurality of suchstructures could then be fitted in series onto a precoiled tube.

What is claimed is:

l. A heat exchanger comprising a coiled thermoconductive tube forconfining a first fluid,

a heat exchange matrix comprising a plurality of thermoconductiveelements, each having substantially planar and substantially parallelopposed major surfaces, in thermoconductive contact with the outersurface of said tube, each element having a major dimension extendingoutwardly from said outer tube surface, and a major dimension extendingsubstantially parallel to the axis of said tube, and a smallestdimension, and

structure directing a second fluid along the outer surface of said tubein an overall flow direction substantially parallel to the said smallestdimension of said elements,

said thermoconductive elements being spaced apart in the axial directionto define open areas therebetween perpendicular to the flow direction,being spaced apart and staggered in the flow direction such thatupstream elements overlap downstream elements, and being arranged withall said planar surfaces in parallel in two sets having elementsextending in opposite outward directions from said tube with elementsextending in the same outward direction from said tube terminating inthe same plane, said plane being parallel to the flow direction, and

thermoconductive elements of adjacent windings of said coiled tubeextending toward and terminating close to one another to definetherebetween at most a narrow passage.

2. The device of claim 1 wherein said coiled configuration defines ahollow heat exchange member of circular cross section.

3. The device of claim 2 wherein at least two said hollow heat exchangemembers are arranged in concentric, nested relationship, with theirforming tubes connected to define a continuous flow path for said firstfluid through saidmembers.

4 he device of claim 3 wherein said members are of cylindricalconfiguration, and said second fluid is directed substantially radiallyoutwardly of said cylinders.

5. The device of claim 1 wherein said tube is helically coiled.

6. The device of claim 1 in the form of a liquid heater, wherein saidsecond fluid is hot gases, and said first fluid is an entering liquidheated by said second fluid in passage through said tube.

7. The device of claim 6 in the form of a vapor generator, wherein saidcoiled tube has a feed liquid inlet at one end and a superheated vaporoutlet at the opposite end.

1. A heat exchanger comprising a coiled thermoconductive tube forconfining a first fluid, a heat exchange matrix comprising a pluralityof thermoconductive elements, each having substantially planar andsubstantially parallel opposed major surfaces, in thermoconductivecontact with the outer surface of said tube, each element having a majordimension extending outwardly from said outer tube surface, and a majordimension extending substantially parallel to the axis of said tube, anda smallest dimension, and structure directing a second fluid along theouter surface of said tube in an overall flow direction substantiallyparallel to the said smallest dimension of said elements, saidthermoconductive elements being spaced apart in the axial direction todefine open areas therebetween perpendicular to the flow direction,being spaced apart and staggered in the flow direction such thatupstream elements overlap downstream elements, and being arranged withall said planar surfaces in parallel in two sets having elementsextending in opposite outward directions from said tube with elementsextending in the same outward direction from said tube terminating inthe same plane, said plane being parallel to the flow direction, andthermoconductive elements of adjacent windings of said coiled tubeextending toward and terminating close to one another to definetherebetween at most a narrow passage.
 2. The device of claim 1 whereinsaid coiled configuration defines a hollow heat exchange member ofcircular cross section.
 3. The device of claim 2 wherein at least twosaid hollow heat exchange members are arranged in concentric, nestedrelationship, with their forming tubes connected to define a continuousflow path for said first fluid through said members.
 4. The device ofclaim 3 wherein said members are of cylindrical configuration, and saidsecond fluid is directed substantially radially outwardly of saidcylinders.
 5. The device of claim 1 wherein said tube is helicallycoiled.
 6. The device of claim 1 in the form of a liquid heater, whereinsaid second fluid is hot gases, and said first fluid is an enteringliquid heated by said second fluid in passage through said tube.
 7. Thedevice of claim 6 in the form of a vapor generator, wherein said coiledtube has a feed liquid inlet at one end and a superheated vapor outletat the opposite end.